In large-capacity power conversion devices, the converter output is high voltage or large current, and therefore, many large-capacity power conversion devices are configured with a plurality of converters multiplexed in series or parallel. It is known that multiplexing converters can not only increase the converter capacity, but also reduce harmonics contained in an output voltage waveform by synthesizing outputs, and as a result, can reduce harmonic current flowing to a grid.
There are various methods for multiplexing a converter: reactor multiplexing, transformer multiplexing, direct multiplexing, etc. In a case of transformer multiplexing, since an AC side is isolated by transformers, there is an advantage that common DC current can be used among the converters. However, there is a disadvantage that, in a case where output voltage is high, the configuration of the multiplexed transformer is complicated and the cost of the transformer increases.
Considering the above, as a power conversion device that is suitable for high-voltage usage and does not require a multiplexed transformer, a multilevel converter is proposed in which outputs of a plurality of converters are connected in cascade. One example of such multilevel converters is a modular multilevel converter.
The modular multilevel converter (hereinafter, referred to as MMC) is composed of arms in each of which a plurality of unit converters called cells (hereinafter, referred to as converter cells) are connected in cascade. Each converter cell includes a plurality of semiconductor switches and a DC capacitor, and through ON/OFF control of the semiconductor switches, outputs both-end voltage and zero voltage of the DC capacitor.
In a case of a three-phase MMC, such an arm is individually formed for each phase, output ends of half the total number of converter cells connected in cascade are used as the AC terminal, both ends of the arms of the respective phases are connected to each other, and their respective terminals are used as the DC terminal. Each phase arm is composed of two arms, i.e., a positive arm and a negative arm. Since each converter cell output of the MMC converter is connected to both sides of the DC terminal and the AC terminal of the MMC converter, each converter cell has a characteristic of outputting both DC current and AC current.
Since the MMC converter is connected to both sides of the DC terminal and the AC terminal, it is necessary to handle a fault that occurs at each terminal. In particular, when a fault has occurred at the DC terminal, power transmission stops until the fault is removed. Thus, the fault needs to be removed immediately. Such a DC line fault includes a DC short circuit fault which is a short circuit between DC lines. In order to suppress fault current that occurs during the fault, a control method has been disclosed in which: an MMC converter including converter cells each formed by semiconductor switching elements in full-bridge configuration is used, and the converter is controlled so as to output voltage against arc voltage that occurs during occurrence of a short circuit (for example, see Patent Document 1).